Creating an Open Price Item
You may want to have an option to enter a variable price into your POS.
You can set this up by creating a product as usual and selecting Open Price Item from the Advanced Pricing menu.
Select "Open Price Item" from this dropdown.
When doing this from the corporate menu don't forget to schedule a menu push to make this product live in all stores.
It is common for stores to want an open price item with sales tax, and one without tax.
To acheive this, simply create two open price items and select the appropriate tax option.
On the POS
On the POS your Open Price item will appear like any other item. However when the button is tapped a screen will appear prompting employees to enter a variable amount. 
Enter the desired amount, hit done, and this product will be added to the cart.